A rigid endoscope that uses a relay lens includes an objective lens and a relay lens. The relay lens transmits an image formed by the objective lens toward the proximal end side of the objective lens. By watching light beams that exit from the relay lens through an eyepiece, an object can be directly observed by a human eye. Alternatively, light beams that exit from the relay lens are formed again on an image pickup device by an imaging lens, thereby the obtained image is displayed in a display device for observation.
A relay lens proposed by Hopkins, in which rod lenses are used, is widely used. This Hopkins type relay lens has advantages in that a bright image is obtained, chromatic aberration is corrected, and asymmetric aberration is also corrected. However, with the Hopkins type relay lens, curvature of field remains. Accordingly, when the number of relay sets is increased or the numerical aperture is increased, effects of curvature of field are increased. This may degrade performance. In order to address this, NPL 1 discloses a technology in which the Hopkins type relay lens is modified so that curvature of field is reduced.
Also, in order to reduce curvature of field of a relay lens, PTL 1 discloses a technology, in which an optical system including two triplets combined together is used as a relay set of a relay lens. Here, each of the triplets includes lenses having positive, negative, and positive refractive powers. With this technology, curvature of field is reduced by an effect produced by the negative lens of the triplet.
In the relay lens disclosed in PTL 1, the triplet design is used to correct curvature of field with the negative lens included in each of the triplets. It is described in PTL1 that, with this negative lens, not only curvature of field caused by the triplet itself but also curvature of field caused by other relay sets using a related-art relay lens can be corrected. However, PTL 1 discloses only the appearance of the triplet and does not disclose a numerical embodiment or aberration. Thus, it is not clear that to what extent the triplet can correct curvature of field caused by the other relay sets of the relay lens. The refractive power of the negative lens needs to be increased for correcting curvature of field within the triplet. In addition, in order to also correct curvature of field caused by the relay lenses of the other relay sets, the refractive power of the negative lens needs to be further increased. The further increase in the refractive power of the negative lens increases spherical aberration, astigmatism, and axial chromatic aberration. This may lead to degradation of imaging performance. In addition, the relay lens becomes more sensitive to decenter. This may lead to an increase in the manufacturing cost. Thus, a further increase in the number of the relay sets of the related-art relay lens used in other relay sets is difficult.